Abstract : The structure of the (100) surface of the orthorhombic Al13Co4 quasicrystalline approximant is described based on a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) study. We discuss the interplay between the surface structure and the cluster substructure of this approximant as well as possible off-stoichiometry effects that could explain the discrepancies found in the literature on possible surface models.